This invention relates to a method of wetting metals with mercury and other liquid metals.
The wetting of metal surfaces with liquid metals has far-reaching practical significance. Wetted metals are useful in the development of high power and high current switches which are important in pulse power applications such as lasers, fusion, isotope separation, intense ion beams, etc. High current switches are used in applications such as metal deposition of optical lenses. Liquid metals are also employed as contacts in rotating machinery, stationary and sliding liquid metal contacts, high energy storage devices, and mirrors used in surveying and mapping.
In high power applications, a switch should be able to withstand high voltages of from 10 kV to hundreds of kV before switching. The switches should also be capable of high current conduction (from tens to hundreds of kilo-amperes) and high coulomb (charge) transfer per shot after switching. The switches should perform reliably over many operations and have a high repetition rate.
Unfortunately, high current switch conduction results in severe erosion of the electrode materials. Switching devices have a short lifetime at normal operating levels because the voltage hold-off drops significantly due to electrode erosion. However, oblation and melting of the electrodes can be reduced or prevented if the electrodes are coated with a material such as mercury or other liquid metals. When the switch conducts, the needed carriers are provided by the vaporized layer of liquid metal. This prevents oblation of the switch electrodes and extends their lifetime.
Energy storage capacitors which are capable of storing very high energies are required for various pulse power applications and for other uses as well. Typically, these capacitors consist of electrodes separated by an insulating medium. The stored energy can be increased by increasing the voltage difference between the two electrodes. This, in turn, increases the stress between the electrodes. If the stress exceeds certain critical levels, a breakdown of the gap results. Protrusions on the electrode surface result in stress enhancement above the average stress in the gap. Therefore, it is essential to have smooth electrode surfaces. Such surfaces can be obtained by coating the electrodes with a liquid metal, resulting in capacitors having increased energy storage capability.
Mercury coated surfaces are useful in surveying and mapping. Mercury pools are used as horizontal mirrors in astrolabes and photographic zenith tubes. The containers commonly used are made of copper and tin-coated copper and as a result, the mercury pool must be frequently cleaned due to contamination. Therefore, ideal containers are those which have been wetted with mercury and which do not introduce any contaminants into the pool.
Liquid metals are also useful in heat pipes, where they efficiently conduct heat. Since mercury is not easily corroded and has a high heat transfer rate (between 200.degree. and 400.degree. C.), it is well-suited for use as heat pipe fluid.
Mercury and other liquid metals may also be employed in sliding liquid contacts and high current switches used in such applications as vacuum metal deposition of optical components. Wetting the switch contacts with liquid metals reduces power losses due to contact resistance. Mercury and other liquid metals, particularly sodium and potassium, are also utilized as slipring current collectors in homopolar machines. Power losses resulting from solid metal contact resistance can be reduced by wetting the metal surface with the liquid metal.
It is known that mercury can wet not only metals which are soluble in mercury, such as platinum, silver and copper, but also metals which are insoluble in mercury, such as iron, nickel, molybdenum and tungsten. Barlow et al., "The Wetting of Metal Surfaces by Liquid Mercury," Vol. 60 No. 10, Zeitschrift Fhur Metalkunde 817-20 (1969). As discussed by Barlow et al., it is believed that a solid surface is wetted by a liquid when the advancing contact angle, .theta., is zero or very neary zero. The contact angle refers to the angle measured through the liquid between the plane surface of the solid and the tangent to the liquid drawn from the point of intersection of the solid, liquid and vapor phases. When a liquid wets a solid surface and spreads upon it at a rate determined by its viscosity and the surface roughness, then .theta.=0.
Barlow et al. wetted various metals with mercury by mechanically and chemically cleaning and polishing the metal surfaces, further cleaning the metals by bombarding the surfaces with argon ions, and then dropping mercury onto the surface of the metal. Because this method did not achieve the desired degree of wetting, Barlow et al. followed the above-recited procedure by further bombarding the mercury-covered metal with argon ions. However, if the time that elapses between the cessation of the argon ion bombardment and the delivery of mercury onto the substrate surface increases, the contact angle increases significantly. Moreover, even when the second ion bombardment immediately follows the addition of the liquid metal to the surface of the metal substrate, the substrate is wetted only around the rims of the mercury drops, and not the entire surface of the metal covered by the liquid metal drop, since the ion bombardment is blocked by the drops themselves and the substrate cannot be further etched beneath the drops. Thus, when the metal substrate is shaken, the drops fall off and wetting is observed only at the junction of the mercury drops and the vapor phases.
It is an object of the instant invention to provide a new and improved method of wetting metals with mercury and other liquid metals which covers the entire surface of a metal substrate with a durable liquid metal coating.